The following is not an admission that anything discussed below is prior art or part of the general knowledge of people skilled in the art in any country.
Immersed membrane water treatment systems include, for example, wastewater treatment systems, such as membrane bioreactors, and water filtration systems, for example intended to produce potable water. Such systems may use air or other gases bubbled from under or between the membranes to scour the surface of the membranes to sustain the permeate flowrate for a given transmembrane pressure. The airflow rate is typically constant for a particular installation when expressed as a volume of air per unit membrane area per unit of time. For any of a variety of reasons, the ideal airflow rate at any moment can be significantly different than the normal rate. These reasons may include:
a) changes in permeate flowrate and hence the loading rate of suspended solids onto the membrane surface;
b) changes in water viscosity:                i) in wastewater systems, whenever sludge is wasted, or if equalization occurs in the membrane tank;        ii) in drinking water systems, if changes in coagulant dosage are necessary because of changes in feed water composition or if changes in recovery occur; or,        iii) in all systems if water temperature changes for example due to seasonal variations;        
c) changes in inlet blower air temperature or density which will affect the mass of air delivered to the membranes for scouring; or,
d) in wastewater systems, changes in sludge filterability due to process changes.
The permeate output from a water filtration system can vary for any number of factors. In municipal applications, factors include the time of day, weather conditions and seasonal fluctuations. In industrial systems, in addition to these factors, production schedules, strikes and plant shutdowns can result in changes in system output.
In wastewater treatment systems in particular (e.g. membrane bioreactors), the influent flows can be highly variable and equalization is generally provided by the system designer. In some installations, equalization is provided upstream of the membrane bioreactor in a separate tank with transfer pumps and in other installations, equalization is provided in the membrane bioreactor tank. In all applications, the viscosity and filterability of the biomass will vary due to process conditions. For example, after sludge is wasted from a bioreactor and fresh feed is introduced, the suspended solids concentration will decrease. In those system designs wherein equalization is provided in the membrane bioreactor, the viscosity will change as the feed flow to the membrane bioreactor varies. When the bioreactor liquid level is near its maximum, the viscosity will be the lowest and when the bioreactor is near its minimum, the viscosity will be at its highest. Sludge filterability will change for any of a variety of conditions including types of biological organisms present, production of extra cellular materials, pH, food to microorganism loading rates (F:M ratio), sludge age, and hydraulic retention time.
Membrane treatment systems consist of any number of separate blocks of membranes, referred to as trains or banks, which collectively produce the required total flow. The output from individual trains can vary as the system output varies for the reasons described previously. In addition, the output from individual trains can be affected by other factors, in particular, the number of trains actually in service (some trains may be out-of-service for maintenance or other reasons) and the degree of fouling of the membranes (if severe enough to limit production from an individual train).
In all membrane treatment applications, there can be defined a “suspended solids mass loading rate”. This rate reflects the rate at which suspended solids are brought to the membrane surface by the feed and is calculated as the “suspended solids concentration×the permeate flux” with units of “mass/unit membrane area−time”. At equilibrium conditions, the rate at which suspended solids are brought to the membrane surface has to equal the rate at which the turbulence and air scouring effects remove the suspended solids.
In control systems for currently manufactured immersed membrane systems, the practice is to set the aeration rate at a fixed rate based on standard designs or pilot data. During commissioning, some manual optimization may result in a change in aeration rates. Once the commissioning is completed, adjustments to the aeration rates are generally not performed. The aeration rate (m3 of air per m2 of membrane area) is typically at or near the optimum aeration necessary when operating at full capacity or at the highest fluid viscosity and is constant for all the trains in a system.
In immersed membrane treatment systems, the membrane filter is immersed in an open tank containing the solution of fluid to be filtered. Filtration is achieved by drawing water to the inside of membrane fiber under a vacuum. The filtered water, also called permeate or filtrate, is transferred to a downstream tank, reservoir or receiving stream. The materials that do not pass through the membrane, including suspended solids, colloids and biological materials, are discharged as a solution called the reject or retentate. This reject can be discharged either continuously or intermittently depending on the system design. Air or other gases, under a slight positive pressure, are provided to the region of the tank under or near the bottom of the membrane filters. The rising gas bubbles scour the membrane surface to reduce fouling and maintain or slow a decline in permeation rate.
The productivity of an immersed membrane system is directly dependent upon many factors including: differential pressure across the membrane (also called transmembrane pressure), the membrane material and the water's mass transfer rate through the boundary layer at the surface of the membrane. The rising air bubbles create turbulence and cause an upflow of water and the combination of turbulence and the upflow of water reduces the boundary layer thickness and increases the mass transfer rate through the boundary layer. The air can be supplied continuously, cyclically (e.g. 10 seconds on, 10 seconds off) or intermittently (e.g. 60 seconds every 30 minutes). Energy is required to provide this aeration and this can be a significant contributor to the overall energy consumption of an immersed membrane system.
At the surface of the membrane filter, a “boundary layer” exists and all water passing through the membrane must first pass through this boundary layer prior to reaching the membrane's surface. This boundary layer is the thin region at the surface of the membrane where a steep velocity gradient exists and the thinner the boundary layer, the steeper the velocity gradient and the higher the mass transfer rate will be through the boundary layer. The thickness of the boundary layer varies with many factors including viscosity and the velocity of the fluid passing over the surface and the concentration of the materials rejected by the membrane. The air supplied near bottom of the membrane induces turbulence and the higher the air flow rate, the thinner the boundary layer.
Membrane bioreactors (MBR) combine membrane technology and activated sludge biodegradation processes for the treatment of municipal and industrial wastewater. In MBR, immersed or external membranes are used to filter the activated sludge from a bioreactor to produce a high quality effluent. Sample MBRs and their operation are described in International Publication No. WO 2005/039742 A1 which is incorporated herein, in its entirety, by this reference to it.
The membranes may be generally arranged in modules or elements which comprise the membranes and the headers attached to the membranes and may be formed together into cassettes and then trains. The modules are immersed in a tank containing activated sludge. A transmembrane pressure in applied across the membrane walls which causes filtered water to permeate through the membrane walls. Solids are rejected by the membranes and remain in the tank to be biologically or chemically treated or drained from the tank for recycle or further treatment.
A typical treatment cycle comprises two stages. The first stage, known as permeation, involves the production of membrane permeate through the application of transmembrane pressure, as described above.
The second stage involves the removal of solids from the membrane pores and surface. Two different operational procedures available are relaxation and backwash. Relaxation is performed by eliminating the transmembrane pressure which causes the permeate production to stop and allows for the air bubbles to remove the sludge particles deposited on the membrane surface. The other operational procedure available for solids removal is backwash. Backwash is performed by reversing the direction of the permeate flow which allows for the removal of the sludge particles loosely deposited on the membrane pores and surface.
A cyclic air flow may be applied to the modules to minimize sludge particle deposition on the membrane surface. The cyclic aeration system uses a valve set and a valve set controller to connect an air supply to a plurality of distinct branches of an air delivery network. The distinct branches of the air delivery network are in turn connected to aerators which may be located below the membrane modules. While the air supply is operated to supply a steady initial flow of air, the valve set and valve controller split and distribute the initial air flow between the distinct branches of the air distribution system such that the air flow to each branch alternates between a higher flow rate and a lower flow rate in repeated cycles. The relative duration of periods of higher and lower flow rate applied to a given aerator are determined by the aeration frequency factor (A.F.F) which can be obtained by dividing the durations of the period of higher air flow by the total duration of the aeration cycle (i.e. duration of higher air flow period plus duration of lower air flow period) respectively. In practical applications, values between 0.25 and 1 are common. For example, a system having four branches may be alternated between states of (a) providing air continuously to all four branches, (b) providing air cycles of 10 seconds on and 10 seconds off by switching between pairs of the branches, (c) providing a cycle of 10 seconds on and 30 seconds off by providing air to each branch sequentially or (d) be at a continuous air off state. The number of air blowers used in state (b) may be twice that of state (c) and the number of air blowers in state (a) may be twice that of state (b). An apparatus and method for providing cyclic air flow are described in U.S. Pat. No. 6,550,747 which is incorporated herein, in its entirety, by this reference to it.
Air bubbles are introduced to the tank through aerators which may be mounted below or within the membrane modules and connected by conduits to an air blower. The air bubbles rise to the surface of the membrane tank and create an air lift which recirculates mixed liquor in the tank around the membrane module. When the rate of air flow is within an effective range, the rising bubbles and mixed liquor agitate the membranes to inhibit solids in the mixed liquor from fouling the membrane pores. Further, there is also an oxygen transfer from the bubbles to the mixed liquor which, in wastewater applications, provides oxygen for microorganism growth if desired.
Chemical cleanings may also be applied in order to remove those foulants that accumulate on the membrane pores despite the routine application of bubbles, relaxation or backwash. Maintenance chemical cleaning, which requires a less concentrated chemical solution, may be applied to maintain or reduce a rate of decline in membrane permeability. Recovery chemical cleaning, which requires a more concentrated chemical solution, may be applied at a lower frequency to restore membrane permeability when it has fallen considerably.
Membrane fouling is probably the most common operational problem encountered in MBR. Membrane fouling occurs when membranes pores are obstructed resulting in the loss of membrane permeability, which is the volume of permeate that can be passed through a membrane surface per unit of pressure or vacuum applied.
The complex mechanisms behind membrane fouling have been widely studied in recent years.
Membrane fouling is highly influenced by diverse MBR operational parameters such as influent wastewater temperature, membrane aeration frequency factor, membrane aeration flow; permeate flux, permeation duration, backwash flow and duration, relaxation duration, maintenance and recovery chemical cleaning frequencies.
The resistance in series method has been used for membrane fouling quantification and identification of the main fouling mechanism (i.e. pore blocking, cake filtration) at any given set of operational conditions. This method allows for a detailed breakdown and quantification of membrane fouling which makes it possible to identify the causes of membrane fouling.
As it has been previously described, there are several operational alternatives for fouling removal available in MBR such as relaxation, backwash, maintenance and recovery chemical cleaning. The application of each of these methods is aimed at the removal of different kinds of fouling. Relaxation and backwash are designed to mechanically remove the foulants deposited on the membrane surface or loosely inserted into the membrane pores. On the other hand, maintenance and recovery chemical cleaning are meant to chemically remove the foulants deeply adsorbed into the membrane pores and biofilm strongly attached to the membrane surface.
Ideally the decision for the application of any of these different fouling removing methods as well as the remaining MBR operational parameters is preceded by a detailed analysis of the membrane fouling and the identification of the main fouling mechanism. However, this analysis, if done at all, is based on off-line data and takes place sporadically or only during piloting or start up. Currently, MBR process control is limited and lacks flexibility to adjust to the different operational conditions encountered in practice. The operational changes are made manually from off-line data and infrequently, if at all, and are highly dependent on the skill and good judgment of the operator.